View more at . In this lesson, you will learn what a rational expression is in algebra and how to simplify rational expressions. A rational expression is any expression that can be written as a ratio of two polynomials. In a similar way, a rational number can be written as a fraction with numerator and denominator, a rational expression can be written with a numerator that is a polynomial and a denominator that is also a polynomial. Here we focus on how to simplify these types of expressions, which usually involves factoring the numerator and denominator and canceling common terms.